The present invention relates to a rubber composition having a base of coupled and/or starred diene polymers comprising silica as reinforcing filler, which can be used for the manufacture of tires.
Since fuel economy and the necessity of protecting the environment have become a priority, it is desirable to produce polymers having good mechanical properties and a hysteresis which is as small as possible, in order to be able to use them in the form of rubbery compositions which can be employed for the manufacture of various semi-finished products entering into the constitution of tires, such as, for instance, under-layers, calendering or sidewall rubbers, or treads and to obtain tires having improved properties, in particular reduced resistance to rolling.
In order to achieve this, numerous solutions have been proposed which consist, in particular, or modifying, inter alia, the nature of the diene polymers and copolymers for polymerization by means of coupling or starring agents or functionalizing agents. All these solutions are concentrated essentially on the use of polymers modified with carbon black as reinforcing filler in order to obtain good interaction between the modified polymer and the carbon black, since the use of reinforcing white fillers, and in particular of silica, has proven unsuitable due to the low level of certain properties of such compositions, and therefore of certain properties of tires using these compositions. Furthermore, the use of silica raises difficulties in working due to silica/silica interactions which tend in raw state to cause an agglomeration of the silica particles before and even after mixing and thus to increase the consistency of the rubber compositions, and in any event to make the working more difficult than the working of carbon black. By way of illustrative examples of this prior art, we may cite U.S. Pat. No. 3,135,716, which describes the reaction of living diene polymers at the chain end with a polyfunctional organic coupling agent in order to obtain polymers having improved properties; U.S. Pat. No. 3,281,383 which, as coupling or grafting agents of living diene polymers, describes polyepoxides, polyisocyanates, polyimines, polyaldehydes, polyketones, polyanhydrides, polyesters and polyhalides, including polyhalogenated compositions of silicon; U.S. Pat. No. 3,244,664 which describes tetraalkoxysilanes as coupling and/or starring agent for diene polymers; French Patent 69 24444, which describes as starring agent compositions comprising either the --O--C(O)--O-- group or the --O--C(O)--R--C(O)--O-- group and which leads to diene polymers having improved working properties and properties in vulcanized state. U.S. Pat. No. 4,185,042 describes diene polymers coupled or starred by means of silicon compounds comprising an epoxide group, the coupling and/or starring agent being present in an amount of 0.05 to 4 moles per mole of living polymer or copolymer having the properties both of an elastomer and of a thermoplastic. There is no reference therein to filled mixtures and nothing permits the person skilled in the art to believe that such polymers could be of interest for compositions comprising silica as reinforcing filler.
The interest in using silica as reinforcing filler in elastomer compositions has existed for a long time, but the use of silica as reinforcing filler has remained very limited due to the drawbacks mentioned above, and this despite the attempts made to remedy the deficiency of the properties exhibited by such compositions. Thus, it has been proposed to use functionalized diene polymers instead of non-functionalized polymers. By way of example of such prior art, mention may be made of European Patent Application EP 0 299 074, which describes a silica-filled diene rubber having a base of a diene polymer functionalized by means of a silane having a non-hydrolyzable alkoxy radical. European Patent Application EP 0 447 066 describes a silica-filled composition containing a diene polymer functionalized by means of a halogenated silane compound. Despite the improvement in the properties obtained with use of such functionalized polymers, the silica compositions described in this prior art have not proved usable for forming the treads of tires, since the properties exhibited, although improved, nevertheless remain insufficient with respect to the level required. This interest in silica-reinforced compositions has recently been reawakened with the publication of European Patent Application EP 0 501 227 which discloses a sulfur-vulcanizable rubber composition obtained by thermomechanical working of a conjugated diene copolymer and an aromatic vinyl compound prepared by solution polymerization with 30 to 150 parts by weight to 100 parts by weight of elastomer of a special precipitated silica obtained by the process forming the object of European Patent Application EP 0 157 703.